Beginner- Bevel Question
You change the scale of the object, right? It will make the whole object being transformed. While bevel, as an operator, only work in mesh edit mode. It wont consider the scale of the object. (Ctrl+A can apply the scale) 



And almost every operator(Redo Last operator) will give you a floating dock to tweak your option in the left-bottom corner. If it doesn't appeared, try hitting F9 (F6 for Blender 2.79 and below).



And if you want more control over the bevel, try out bevel modifier:



You can custom a lot of effect with edge weight, one should always consider using modifier unless the task is not complicate and don't need further modification.